VVT (Variable Valve Timing) has conventionally been known that changes the phase (crank angle) in (at) which an intake valve or an exhaust valve is opened/closed, according to an operating condition. Generally, the VVT changes the phase by rotating, relative to a sprocket or the like, a camshaft that causes the intake valve or exhaust valve to open/close. The camshaft is rotated by such an actuator as hydraulic or electric motor. Particularly, in the case where the electric motor is used to rotate the camshaft, the torque for rotating the camshaft is difficult to obtain, as compared with the case where the camshaft is hydraulically rotated. Therefore, in the case where the electric motor is used to rotate the camshaft, the rotational speed of the output shaft of the electric motor is reduced by a speed reducer mechanism or the like, thereby rotating the camshaft. In this case, the degree of phase shift is restricted by the speed reducer mechanism.
Japanese Patent Laying-Open No. 2004-150397 discloses a valve timing adjustment device with a great degree of freedom of phase shift. The valve timing adjustment device disclosed in Japanese Patent Laying-Open No. 2004-150397 is provided to a transmission system for transmitting drive torque from a drive shaft of an internal combustion engine to a driven shaft for opening and closing at least one of an intake valve and an exhaust valve, so as to adjust the timing at which at least one of the intake valve and the exhaust valve is opened and closed. The valve timing adjustment device includes: a first rotator rotated around a rotation centerline by the drive torque from the drive shaft; a second rotator rotated around the rotation centerline together with the rotation of the first rotor and in the same direction as the first rotor so as to rotate the driven shaft synchronously, wherein the second rotor is rotatable relative to the first rotor; and a control device having a control member and varying the radial distance of the control member from the rotation centerline. The first rotor has a first hole forming a first track that extends so as to vary its radial distance from the rotation centerline. The first hole makes contact with the control member that passes through the first track, with the contact between the first hole and the control member occurring at two sides of the first hole toward which the first rotor rotates. The second rotor has a second hole forming a second track extending so as to vary its radial distance from the rotation centerline and making contact with the control member that passes through the second track, with the contact between the second hole and the control member occurring at two sides of the second hole toward which the second rotor rotates. The first track and the second track slant toward each other along the rotational direction of the first rotor and the rotational direction of the second rotor. In this valve timing device, in the case where the electric motor generates no torque, the phase is maintained.
According to the valve timing adjustment device disclosed in this publication, the first hole of the first rotor forms a first track that extends so as to vary its radial distance from the rotation centerline and makes contact with the control member that passes through the first track, with the contact between the first hole and the control member occurring at two sides of the first hole toward which the first rotor rotates. Furthermore, the second hole of the second rotor forms a second track extending so as to vary its radial distance from the rotation centerline and makes contact with the control member that passes through the second track, with the contact between the second hole and the control member occurring at two sides of the second hole toward which the second rotor rotates. Here, the first track and the second track slant toward each other along the rotational direction of the first rotor and the rotational direction of the second rotor. Therefore, when the control device is to change the control member's radial distance from the rotation centerline, the control member presses against at least one of the first hole and the second hole, whereby the control member passes through both the first track and the second track, and thus the second rotor is caused to rotate relative to the first rotor. In the valve timing adjustment device which operates in the foregoing manner, the degree of phase shift of the second rotor with respect to the first rotor is dependent upon the length of the first track and the second track and the degree to which the first track and the second track slant toward each other. By extending the first track and the second track such that they vary their radial distances from the rotation centerline, relative freedom is achieved in determining the length and the mutual slant of the tracks. In turn, this increases freedom in setting the degree of phase shift of the second rotor with respect to the first rotor, and therefore, the degree of phase shift of the driven shaft with respect to the drive shaft.
However, as done by the valve timing adjustment device disclosed in Japanese Patent Laying-Open No. 2004-150397, if an electric motor is used as an actuator, the electric motor has to be controlled in consideration of power consumption and heat generation for example. Further, since the phase for example corresponding to the most retarded angle is determined depending on the mechanical structure of the VVT, the electric motor has to be controlled so as not to damage the VVT. Japanese Patent Laying-Open No. 2004-150397, however, does not include any description concerning control in consideration of these factors.